This invention relates to the field of optical terminals for receiving optical fibers, and in particular to terminals affording precise control of the exit or output angle of a light beam issuing from an optical fiber.
The out-coupling of a light beam from a fiber at a very precise exit or output angle remains one of the many challenges encountered in the field of fiber optics. Specifically, a light beam traveling through a fiber, e.g., a signal beam, has to be out-coupled for purposes of optical and/or electronic processing. For example, a signal beam may have to be out-coupled and filtered to ensure passage of a particular wavelength, and then converted into an electronic signal for data retrieval and computing purposes. Such situation may arise in the field of fiber optic communications, e.g., in Wavelength-Division-Multiplexed (WDM) networks in which a fiber carries a number of signals at a predetermined set of wavelengths.
Many different types of optical fibers are known and used in fiber optics. A typical fiber has three major components. The first is a waveguide or core, which has a high refractive index. The core is surrounded by a lower index cladding layer which protects the core and prevents light from leaking out. A jacket or reinforcement layer surrounds the cladding layer to protect the optical fiber from external influences and/or to provide additional mechanical support.
When out-coupling a light beam from the core of the fiber, the output angle is very important because it determines the angle at which the light beam is incident on the subsequent optical or electronic component. For example, in the case of a wavelength filter, the angle of incidence will affect the center wavelength transmitted by the filter. In other cases, e.g., where the out-coupled light is to be in-coupled into another fiber, control of the output angle is essential to ensure proper in-coupling and low insertion losses.
A narrow band filter operating on light transmitted between two optical fibers can be made according to a method taught by Si et al. in U.S. Pat. No. 5,612,824. In this invention the filter is sandwiched between two graded index (GRIN) lenses which preferably share the same optical axis. The input and output fibers are positioned at corresponding input and output ports which are initially equidistant from the optical axis by the same amount. Consequently, the light out-coupled from the input fiber passes through the first GRIN lens, is filtered by the filter, and then passes through the second GRIN lens to the output fiber. A shift in the angle at which the light is incident on the filter is achieved by altering the distance of the input port from the optical axis. A corresponding adjustment of the output port is required to ensure proper in-coupling of the filtered light.
In U.S. Pat. No. 5,799,121 inventors Duck et al. teach a multi-port optical device related to the one taught by Si et al. In the device of Duck et al. the output angle of light emitted from an optical fiber and reflected by a filter into a second optical fiber is adjusted by moving the input and output ports. In particular, the distance between the ports and the optical axis of the GRIN lens is varied to obtain angle tuning.
Neither Si et al. nor Duck et al. teach or suggest a suitable method for adjusting the offset distance of the ports from the optical axis to achieve precise tuning of the angle at which the light is incident on the filter. Furthermore, they do not discuss a suitable mechanism for properly holding the fibers at the ports.
Feuer et al. in U.S. Pat. No. 5,857,048 teach a photonics package which can be used in optical communications networks. The signals can be guided by separate fibers. Precise positioning of the fibers with respect to the optical axis of the optical element of the package is achieved by using a dual-fiber ferrule which has a separate bore for each fiber. Alternatively, fibers can be placed in the same bore in a tight fit. This method of adjusting the offset distance of the fibers from the optical axis is contingent on the dimensions of the bore and the standardized diameters of the fibers. Hence, the method is not sufficiently flexible for precise determination of the offset distances.
Another fiber-optic coupler as well as devices and systems incorporating this coupler is presented by Pan et al. in U.S. Pat. No. 5,652,814. Pan et al. suggest that fibers should be placed in a central opening or bore of a sleeve or capillary and that the cladding layers may be used to maintain a distance between the cores. A lensing element, such as a GRIN lens, is positioned in front of the central opening. The cores of the fibers are parallel to and slightly offset from an optical axis of the GRIN lens. Thus, the beams issuing from the fibers exit the GRIN lens at certain angles. The claddings of both fibers can be tapered.
The method of arranging fibers in the sleeve as taught by Pan et al. does not allow one to achieve very precise determination of the offset distance from the optical axis. Hence, very precise control of the output angle from the coupler""s GRIN lens is not ensured. Moreover, the method does not allow a designer to alter the output angles in an easy and straightforward manner.
In light of the above, it is a primary object of the present invention to provide an optical terminal which overcomes the prior art limitations and enables very precise control of the offset distance between a fiber core and an optical axis of a light-guiding element, such as a GRIN lens. This precise control of the offset distance ensures very precise control of the output angle of the light emitted from the fiber core and transmitted through the light-guiding element.
It is another object of the invention to provide a method for placing fibers in an optical terminal at a very precisely defined distance from the optical axis of the light guiding element.
Yet another object of the invention is to ensure that the optical terminal of the invention and the method of placing fibers therein are easy and straightforward to implement.
The above objects and advantages, as well as numerous additional improvements attained by the heterodyne detection system of the invention are pointed out below.
The objects and advantages of the invention are attained by an optical terminal having a capillary with a central opening or a bore. The bore has an insertion opening and a coupling opening. Preferably, the insertion opening is larger than the coupling opening. A light-guiding element having an optical axis is positioned in front of the coupling opening. The light-guiding element may be joined or bonded to the capillary, if required.
A first optical fiber with a first tip, a first core for conducting a light beam and a first cladding surrounding the core is placed in the bore such that the first tip is positioned at the coupling opening. The first optical fiber has a first fitting length along which a portion of the cladding is removed to produce a first adjusted cross section.
A second fiber having a second fitting length and a second adjusted cross section different from the first adjusted cross section is also placed in the bore. The second fiber is used to wedge the first fitting length of the first optical fiber in the bore such that the first core at the first tip is maintained at a first distance from the optical axis of the light-guiding element.
Preferably, the bore""s insertion opening is larger than the coupling opening. The cross section of the coupling opening may differ, e.g., it may be circular, rectangular, rhombic or other. Also, the adjusted cross sections of the first and second fibers are preferably circular.
The light-guiding element may be any suitable type of waveguide or lens. In most applications a graded index (GRIN) lens is used. Other lenses such as ball lenses or micro-drum lenses can also be used.
The second fiber does not have to be an optical fiber. For example, the second fiber can be a reinforcing fiber which serves to wedge the first fiber""s fitting length only. Alternatively, the second fiber can be an optical fiber and have a second core, a second cladding and its second adjusted cross section can extend along the second fitting length. In this situation, the second fiber has a second tip. The second tip is positioned at the coupling opening such that the second fitting length is wedged in the bore and the second core at the tip is maintained at a second distance from the optical axis. The first and second fitting lengths can be equal.
The bore of the optical terminal is preferably filled with an epoxy. The epoxy enters and fills the regions between the bore and the first and second fibers.
Of course, more than two fibers can be inserted into the bore and wedged to ensure a predetermined distance between their cores and the optical axis. Some of those fibers can be optical fibers and some reinforcing fibers, as necessary.
In accordance with the method of the invention an output angle of a light beam from the first tip of the first optical fiber is controlled by accurately setting the first distance from the optical axis. This is ensured by using removing the cladding in the fitting lengths in a controlled manner. The preferred method for accomplishing controlled removal of the cladding is by etching. A uniform etching process ensures uniform removal of the cladding and hence the adjusted cross section in this case is circular.
In order to prevent light leakage from the core and not compromise the mechanical stability of the fiber it is important not to etch the cladding closer than four times the radius of the core.
Further details of the invention are found below in the description with reference to the attached drawing figures.